Whenever you remove or alter something that’s holding the house up. That’s the short answer, and it covers more work than most people expect, including plenty of projects that never go near a planning application. Any architects bromley homeowners work with will raise it early, because the calculations shape the design rather than following it.
Here’s when they’re needed, what they actually produce, and what it costs.
The Simple Test
Ask one question about the work. Is anything currently carrying load being removed, altered, or asked to carry more?
If yes, you need calculations. Planning permission has nothing to do with it. This is Building Regulations, and it applies whether your extension is permitted development or a full application.
Building Control will ask for a calculation pack before signing off structural work. Without one, the work doesn’t get approved, and unapproved structural work is a problem that surfaces at sale.
Work That Always Needs Calculations
Removing a load bearing wall. Including the rear wall of the house when you extend, which every rear extension involves. The beam replacing it needs sizing, and so do the supports at each end.
Any beam or lintel. Steel or timber. Span, section size and bearing all get calculated against the actual load, not selected from a table.
Loft conversions, every type. The existing ceiling joists were never designed to carry a floor. New floor beams, a ridge beam where the roof structure changes, and the structure for any dormer or gable all need designing.
Chimney breast removal. Removing a breast on the ground floor leaves the stack above unsupported, and a gallows bracket arrangement needs calculating rather than assuming.
New openings in existing walls. Wider doors, new windows, knocking two rooms together.
Foundations for the extension itself. Depth and design depend on ground conditions, proximity to trees and what’s already there.
Any double height space or floor void. On a recent Bromley new build the design included a void connecting the kitchen through to the basement level. Openings like that remove a chunk of structural floor plate, and the surrounding structure has to be designed to work without it. Impressive spaces are almost always the ones with the most engineering behind them.
Work That Usually Doesn’t
Non structural partition removal, where the wall carries nothing above it. Worth confirming rather than assuming, because internal walls in some construction types are doing more than they look like they’re doing.
Replacing a window in an existing opening, same size, existing lintel sound.
Internal fit out, decoration, kitchens and bathrooms.
Who Is Actually Qualified to Do This
Worth knowing, because the title isn’t protected in the way people assume.
Look for chartered status through the Institution of Structural Engineers or the Institution of Civil Engineers. The letters are MIStructE, CEng or MICE. Building Control treats calculations from a chartered engineer differently to a set of numbers from someone without that standing, and professional indemnity insurance sits behind the work.
Ask about insurance directly. If a design fails, the calculations are the document everyone goes back to.
What You Actually Receive
The output is a calculation pack, and it typically includes several things.
Load calculations showing what’s being carried and where it goes. Beam sizing and specification, giving the section, grade and required bearing length. Foundation design where new foundations are involved. Connection details where new structure meets old. And a set of structural drawings the builder works from and Building Control assesses.
That last point matters. The calculations aren’t paperwork produced to satisfy a regulation. They’re the instructions the builder uses to buy the right steel and build the right support.
What It Costs in 2026
Fees for extension work generally run £400 to £1,500, with straightforward single beam jobs at the lower end and complex multi element work at the upper.
| Item | Typical 2026 range |
| Single beam, straightforward | £400 to £700 |
| Standard rear extension package | £600 to £1,000 |
| Loft conversion package | £600 to £1,200 |
| Complex or multi element work | £1,000 to £1,500 plus |
Set against a build budget in the tens of thousands, this is one of the smallest line items in the project and one of the least sensible places to economise. An undersized beam is not a cost saving, it’s a defect that gets found by a surveyor at some point in the future.
Why Having the Engineer In House Changes the Design
Most practices outsource this, which means the design goes out, comes back with calculations, and any conflict between what was drawn and what’s structurally sensible gets resolved late.
Extension Architecture keeps a structural engineer in the same team as the design staff, which is uncommon across the industry. The practical effect is that beam spans get tested while the layout is still fluid, rather than after a client has fallen in love with a plan.
That matters more than it sounds. Beam cost rises sharply with span, and a design that removes an entire rear wall needs a considerably larger section than one retaining a pier. Catching that in week two costs nothing. Catching it after planning permission means either absorbing the cost or applying to amend an approved scheme.
The Ground Conditions Question
Foundation design depends on what’s under the site, and this varies enormously across London and the South East.
Standard foundations run around a metre deep. In clay soils, widespread across parts of Surrey and outer South London, that can rise to 1.5 metres or more. Groundworks on a standard single storey extension typically run £3,000 to £8,000, and clay pushes that upward.
Trees complicate it further. Clay shrinks and swells with moisture, and a mature tree within influencing distance changes the required foundation depth substantially. That’s a calculation, not a guess.
Bromley illustrates the point well. BR1 is dominated by Edwardian and 1930s houses on generous, often leafy plots, and mature trees near the rear of a property are common. It’s exactly the housing type where foundation design deserves attention before a builder prices the job. A structural engineer surrey projects rely on will want the same information before designing anything, because the clay conditions across Elmbridge and the wider county behave similarly.
Building Control: Two Routes
You have a choice, and it affects timeline.
The local authority route goes through the council’s own Building Control department. The alternative is an approved inspector, a private body carrying out the same statutory function.
Approved inspectors are often quicker and more flexible on site visit timing. Local authority Building Control has direct access to the council’s records on the property. Either is valid, and the decision is worth making early rather than during the build.
Party Walls Are a Separate Matter
Worth separating, because people conflate the two.
Structural calculations satisfy Building Regulations. The Party Wall Act 1996 is separate legislation about your neighbour’s rights.
Cutting into a shared wall to support new structure triggers notice under Section 3. Excavating near a neighbour’s foundation triggers Section 6. Two months notice for Section 3, one month for Section 6.
Your engineer’s calculations often form part of what a party wall surveyor reviews, but having calculations doesn’t discharge the party wall obligation, and serving notice doesn’t discharge the Building Regulations one. Both apply.
Getting the Order Right
The sequence that avoids rework is fairly consistent.
Survey the existing structure first, including foundations and party wall construction. Establish the planning position. Design the scheme with the structural strategy considered rather than assumed. Get calculations before finalising the drawings. Serve party wall notices early, because a neighbour with two months of warning behaves very differently to one given a fortnight.
Then tender. A builder pricing a scheme with calculations already done is pricing something real. A builder pricing a scheme without them is pricing an assumption, and the difference between those two numbers appears later as a variation.
